High School

Dimerization is a side reaction that occurs during the preparation of a Grignard reagent. Propose a mechanism that accounts for the formation of the dimer.

Answer :

Answer;Dimerization is a common side reaction that occurs during the preparation of a Grignard reagent. The formation of a dimer is a result of the reaction between two equivalents of the Grignard reagent, which can occur via a radical mechanism:

1. Initiation: The reaction begins with the formation of a radical species by the reaction between the Grignard reagent and a trace amount of oxygen or moisture in the solvent:

RMgX + O2 (or H2O) → R• + MgXOH (or MgX2)

2. Propagation: The radical species reacts with another molecule of the Grignard reagent to form a new radical species, which then reacts with a molecule of the solvent:

R• + RMgX → R-R + MgX•

MgX• + 2R-MgX → MgX-R + R-MgX-R

3. Termination: The radical species produced in step 2 can react with other molecules of the Grignard reagent or with other radicals to form larger oligomers, such as tetramers and higher.

2R• → R-R

R• + R-R → R-R-R

R• + R-R-R → R-R-R-R

Overall, this mechanism accounts for the formation of the dimer (R-R) during the preparation of a Grignard reagent. The formation of the dimer can reduce the yield of the desired Grignard reagent, so care must be taken to minimize the amount of oxygen and moisture present in the reaction.

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